fix(posting): mark removed split parts for reclamation

[KeisukeYamashita]

Refs #9824

Description

Write BitEmptyPosting records for split parts removed during rollup, allowing Badger compaction to reclaim their old payloads.

Use the replacement parent's timestamp, including the +1 for finite read timestamps, to preserve historical reads.

Add regression tests covering:

• Partial and whole-list deletion for data, reverse, and index keys.
• Splits created through normal mutations and rollup.
• Historical reads, timestamp boundaries, reinsertion, delayed rollup writes, database reopen, and backup conversion.

Validation: the posting test suite, targeted race tests (three runs), targeted debug tests, and Dgraph build pass. Trunk has not yet been run.

This change does not remove empty-key metadata or retroactively clean up already-orphaned parts.

Checklist

• The PR title follows the Conventional Commits syntax.
• Code compiles correctly and linting (via trunk) passes locally.
• Regression tests added for the bug fix.

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